. This week an article in
magazine on long-term western U.S.
reconstructions using tree rings has set media headlines ablaze across the U.S.:
in the northern Rocky Mountains has shrunk at an unusually rapid pace during the past 30 years, according to a new study. The decline is "almost unprecedented" over the past 800 years, say researcher"
" The decline in recent decades of the mountain snows that feed the West's major rivers is virtually unprecedented for most of the past millennium..."
What this paper shows is the warming of the 20th century and beyond is already affecting and will profoundly affect the frequency of droughts in the West, simply by whittling away at the snowpack.--Phil Mote, Climate Scientist at Oregon State University.
Rockies Shrinking Snowpack Hurts Water Supply: Changes in Rockies Could Leave Million Without Water (CNN)
"Pederson said the findings add further context to studies that indicate a 30% to 60% decrease in the snows that gather at the upper elevations of the mountains. ... "It's a two-part story," he said. "In a nutshell, what you're seeing is synchronous, declining snowpacks across the West since the 1980s."
I could provide a dozen more of these stories...but you get the message. And when you read these articles, virtually all go on to pin the blame on human-induced global warming. And before I go on, let me make clear that I am not a global warming denier, but a mainstream atmospheric scientist who believes human-induced global warming is inevitable and a major threat if we don't deal with.
So what stimulated all these stories? An article in Science Magazine that was released last week:
The Unusual Nature of Recent Snowpack Declines in the North American Cordillera By Pederson et al. Here is the abstract:
In western North America, snowpack has declined in recent decades, and further losses are projected through the 21st century. Here, we evaluate the uniqueness of recent declines using snowpack reconstructions from 66 tree-ring chronologies in key runoff-generating areas of the Colorado, Columbia, and Missouri River drainages. Over the past millennium, late-20th-century snowpack reductions are almost unprecedented in magnitude across the northern Rocky Mountains, and in their north-south synchrony across the cordillera. Both the snowpack declines and their synchrony result from unparalleled springtime warming due to positive reinforcement of the anthropogenic warming by decadal variability. The increasing role of warming on large-scale snowpack variability and trends foreshadows fundamental impacts on streamflow and water supplies across the western United States.
In this paper they looked at tree rings around the mountains of the western U.S. (mainly Rockies) and tied their variations to changes in snowpack. Then they show an 800-year chronology of this snowpack and suggest that the recent loss of snowpack is unprecedented in magnitude across the northern Rocky Mountains and the nature of snowpack variations have changed--that snowpack has changed along the entire north-south distance of the Rockies, rather than the north and south being out of phase as they suggest is normal. They further suggest that they in phase loss of snowpack is a sign of general warming: human-caused global warming.
Connecting snowpack variations with a single meteorological value is a tricky business, since trees can respond to variations in temperature, precipitation, insect damage, and human intervention (e.g., thinning or logging), among other things. And problems with tree-ring interpretation was the centerpiece of the recent Hockey-Stick climate controversy and Climategate. But let us accept for the time being that their snowpack-tree ring variation is completely reliable. Should we still accept that scary snowpack tales?
Has there been unusual loss of mountain snowpack the past few decades that suggest rapid human-induced warming? Well, lets go to one of the figures the authors provided in their supplementary information that shows the snowpack (actually snow water equivalent of the snowpack) for the last century for the three areas they talk about--Colorado, Yellowstone and northern Rockies zones. The black lines are based on real measurements of snowpack, the yellow lines are from the tree rings, and the cyan and dark blue lines are smoothed averages of them.
Well, what do we conclude? No real trend in the high upper Colorado Basin and slight downward trends during recent decades (since roughly 1975) from a maximum period in the 50s and 60s. This modulation is consistent with well-known mode of natural variability--the Pacific Decadal Oscillation. And if human-induced global warming is evident it should be after roughly 1970-1980 since before that time our addition of greenhouse gases were too small to have enough impact to be clearly observable.
What about snowpack variations over the nearby Cascades during the past 80 years?A paper published in the peer-reviewed Journal of Climate (I am one of the co-authors) suggests a fairly benign situation:
Since 1976 the snowpack has been INCREASING, and since 1930 the trend is downward by roughly 23%--and much of that could not have been caused by human intervention. As most climatologists know, the earth started to warm after the late 1800s...coming out of the little ice age....and this variation is almost surely natural. Finally, a recently submitted paper by John Christy of U. Alabama, using more snow records than ever before, indicates little loss of snowpack of California during the past century. (see graph)
And here is plot of Snow Water Equivalent (SWE) for Snotel sites from throughout the western U.S. since 1933:
A very slow, fairly steady decline. Nothing abrupt or scary. No real smoking gun for human-induced global warming.
I can show you a lot more snowpack graphs, but the bottom line is that there has NOT been major snowpack losa during the past few decades in the western U.S. and the long-term trend, starting over a hundred years ago, is gently down with some natural variation on top of that.
Here is long-term snowpack graph from the Science article--going back 800 years! (below) Not much change in Colorado. Yellowstone and the northern Rockies are lower recently than most of the historic record, BUT THIS STARTED AROUND 1900 WELL BEFORE THE HUMAN-INDUCED SIGNAL WOULD BE SIGNIFICANT.
The other point of this paper is that the pattern of snowpack change has altered during the past few decades--which they ascribe to anthropogenic global warming. As shown in their figure (below) the northern Rockies and Colorado snowpack tend to be out of phase, which they suggest is due to movement of the jet stream north and south, forced by El Nino and La Nina, for example. The shaded areas indicate what they considerto be anomalous periods where the whole Rockies have less snowpack simultaneously. 
Now the current period of such anomalous behavior is very short---a very few decades--and that could well be just random noise or natural variability. But just as important, is that there are other periods of similar behavior when global warming could not be the reason. Want to see? Here is an example from the early 20th century (see blue shading)
I hope I haven't bored anyone, but I have spent time on this for a reason. If you really check out the Science paper and look at the data, the loss of snowpack during the past few decades have not been serious. There is no clear smoking gun of anthropogenic global warming.
Folks, we are in the early days of the warming and most of the action is yet to come. We need to be very careful on jumping to conclusions too early, since that only aids the deniers and skeptics who are just looking to pounce on excessive claims.
